Petroleum production by underground combustion



United States Patent Claims. Cl. 166-11) No Drawingu- 10 This invention relate-s to a process for carrying outan underground combustion in an oil-bearing.formation'for the purpose of producing petroleum, in which a part of the combustible'ma'terial (oil, oil residue, tarry substances, etc.) originally present-in the formation is burnt.

.It has long ago been proposed to produce oil by'the use of underground combustion. The great difficulties arising in this method of'production chiefly relateto the maintenance of a regular displacement of the combustion front, starting: the combustion .and re-igniting the oil in' the formation when the combustion has stopped or was stopped for any reason.

A known method of influencing the displacement of the combustion front is to regulate the oxygen content of the mixture ofair and inert gases to be supplied to the formation. T his method has disadvantages because the injection pressure of the gases is-not constant, as a result of which it is timetand againnecessary. to correct the oxygen content of the gas mixture.

Ignition-of the oil in the formation round a well is often eflected by means of hot gases. It may then :happen (usually when the formation contains relatively light oil) that :the oilin'the vicinity ofthe well wholly disappears, e.'-g; ithrough evaporation; sometimes the only material remaining is so difficult to burn-(residue, coke) that the temperature in situ does notrise to the high degree required. for combustion, it being then impossible to effect the required combustion.

'Diflicu'lties of this kind occur when the combustion stops, or is stopped, and has to be restarted later. In this case it has been proposed to force liquid or gaseous fuel throughthe injection well into the formation. This method is not very attractive, particularly when the combustion front is no longer close to the injection well. It is an object of the present invention to provide a method; by the use of which the difficulties outlined can .be avoided andby means of which thecom'bustion can be simply and readily regulated in actual practice. With this method it isnot only possible to obtain a regular displacement-of the combustion front, butthe combustion can-also be ignited and re-ignited thereby. v v

. feordin-g. to the invention, the combustion process an ex'ternal force is exerted once or several times on the oilfin the formationlthatthe pressure gradient prevailing 60 "in the oil 'during combustion reverses its direction, sothat the oil, or atleast a part thereof is enabled to carry out temporarily a movement which is opposite or approximately opposite to the direction inWhich the combustion should normally progress. V 65 'As the combustion zone progresses, the oil in the formation is' 'fo'rced towards the 'part of the formation which is still unheated.- As a 'result'of this congestion of oil, it may. 'b enecessaryto'- use increasingly higher pressures .(especiallywh'en the formation contains-relatively heavy oil) in ordert-o forcethrough the formation the oxygeny-ieldin'g. gases :(usually oxygen airor a mixture ofair Patented Dec, 2, 1958 2, oxygen and'other gases) required for the combustion and the combustion may finally stop.

It has'now been found that the process according to the invention can be advantageously used when such congestion is imminent." If, therefore, the pressure drop over the formation (a measure'of such drop is, for example, the difference in pressure between the injection well and the production Welly shows a continual tendency torise, and especially when thepressure drop is abnormally high, the pressure on the injection well may be conveniently temporarily reduced (if desired to atmospheric pressure "or'even less) and/or 'thepressure on the production-well may be increased, in such a Way that a resultant force is exerted on the oil'in the formation which is opposite or approximately opposite to the direction in which the combustion normally progresses. Since the pressure in the production'well isnow higher than that in the injection well, the oil'is enabled to flow back. After some time has elapseddheformer state'can be restored by forcing air into'the injection well'in the usual way. Model experiments have shown that a' considerable improvement could be obtained in'this-way, so that a regular combustiono'f the oil in the formation was again ensured for a fairly long period-. Sometimes it is necessary to repeat the 'proc'ess'one'or, more times'in' order to overcome the abovementioned undesirable consequences of 'oil-congestion.

The process according to the inventionwill, however, often be used without this necessity since as a result of the reversal'of the pressure inthe formation the combustion is favourably effected in situand takes place more uniformly, while in'additi'on the light hydrocarbon fractions' (produced by evaporation and/or cracking of the oil) which are entrained with the combustion gases are more intimately'contacte'd with the oil to be produced and thus reduce its viscosity. This action facilitates 'the propulsion of the oil and a higher oil production can be obtained.

The present process can also be successfully used when the ignition and particularly the re-ignition of the oil in the formation gives'rise to difiiculties. The oil present in the formation at some distance from the well, or the combustion zone, is enabled to flow back to the vicinity of the injection well, or the combustion zone, respectively, by the reversal of the pressure gradient. It has been found that the oil can usually be re-ignited by normal injection of air, it being desirable to use a subatmospheric pressure in the injection well, so that'the smallest possible amount of oxygen-free combustion gases remain in the part between injection Well and combustion zone.

If, in a special case, the temperature of the oil in the formation. has dropped too much for it to be re-ignited by injecting'ai-r, hot combustion gases may, for example, first be' blown in, whereupon the process according to the invention may be used. f

I ln'carrying out an underground combustion the phesomenen foften'occurs of the oil (the viscosity of which hase become low through heating) escaping in front of the combustion zone, so that the combustion threatens to stop through a lack of sufficient combustible material. The" decrease in combustion intensity can be recognized by analyzing the combustion gases produced 1 (measuring CO content, CO content, spectrum of the cracked gases, etc.')'. If this a'nalysis points to a continual decrease in combustion intensity, particularly when this intensity assumes a very low value, the present process may also be used to advantage.

In order to bring about the reversal of the pressure gradientin the formation, the production Well (or wells), may, for example, be'brought under pressure, or placed under a higher pressure than was already prevailing in thewell. This can usually be done by means of air. Sometimes, however, it is advisable to use an inert gas (e. g. a hydrocarbon gas) for the purpose as in this case oxidation of the oil is prevented.

By reversing the pressure gradient the combustion usually temporarily stops. Only when gases giving off oxygen are blown through for a sufiicient time will combustion be able to take place again even during the period in which the pressure gradient is reversed.

The reversal period should not be too long, so that the oil is not forced back in too great a quantity or too far.

It is often advisable that the pressure gradient in the opposite direction should be greater than that prevailing during the normal combustion process.

The underground combustion is usually carried out by means of one or more injection wells and one or more production wells which lead into the oil-bearing formation at some distance from each other. It is, however, also possible to use a single well in which injection and production occur at different heights in the oil-bearing formation.

If two (or more) production Wells are available, the process can be carried out in such a way that one well is reserved for the actual production, the other being used to exert the external force on the oil in the opposite direction (the first well then being closed). In this case the return movement of the oil will often not be exactly opposite to the direction in which the combustion front is usually displaced, but will deviate somewhat therefrom. In practice operations are also often carried out in such a way that the underground combustion is only applied to a part of the field in order to develop heat in situ, after which water is injected. The steam thus generated (and possibly the hot water following) displaces the oil from another part of the field thus promoting its production. The present process can of course also be advantageously used in this embodiment of the combustion.

The invention will be further illustrated by means of two model tests. The two tests were carried out in a vertical iron tube 2 metres long and 8 cm. in diameter. The tube was able to resist a super-atmospheric pressure of 10 atm. and was also insulated in order to minimize heat losses.

The lower part of the tube was provided with a valved line for supplying air and the upper part with a line for drawing off the combustion gases and the gaseous and liquid products produced which were collected in a vessel communicating with this latter line. The liquid products were drawn off from this vessel at the lower part and the gases at the upper part via valved lines. The tube was filled with silver sand mixed with crude oil. The combustion of the oil in the sand was started by means of an electric heating element arranged in the lower part of the tube. The temperature in the tube was measured by means of thermocouples.

A. Silversand with a permeability of approximately 30 darcys was mixed with a quantity of Tia Juana crude oil (viscosity 6500 cs. at 25 C.) the tube was filled with the oil sand thus obtained. The porosity of the oil sand was 37% (i. e. 63% consisted of sand). The degree of saturation was 42% (i. e. 42% of the originally empty space between the sand particles was filled with oil and 58% with gas).

The flow rate of the air at 10 atm. super-atmospheric was 650 litres per hour (measured under standard conditions). After the formation of a combustion front at a temperature of approximately 600 C. there was first a pressure drop over the tube of 1 atm. The counterpressure on the outlet side was therefore 9 atm. gauge at the start. The pressure drop over the tube increased in 75 minutes to approximately 10 atm. (at a constant flow rate of air); the gas velocity began then to fall.

The draw-off on the upper part of the tube was then closed, causing the pressure at this place to rise to approximately atm. gauge. The pressure on the inlet side was then decreased until atmospheric pressure was reached. This caused a reversal of direction of the pressure gradient in the oil sand. This state was maintained for 5 minutes.

The injection pressure of the air was afterwards again increased to 10 atm. gauge and it was again possible to bring the gas stream on the outlet side to 650 litres per hour by regulating the valve. The pressure drop over the tube was now only 4.5 atm.

The increase in the pressure drop over the tube at 10 atm.. was clearly caused by the very viscous oil being forced towards the cold upper part of the tube. By using the process according to the invention this pressure drop could be reduced to a much lower value, so that the combustion of the oil could be restarted.

After approximately anhour the drop in pressure had again increased to 10 atm. The above procedure was then repeated, except that the reversed pressure gradient was now maintained for 15 minutes. As a result the pressure'drop over the tube was reduced to V2 atm.

During the further course of the test the process was again repeated. In this way it was possible to maintain a substantially constant combustion of the oil in the tube.

In all 56% of the oil present in the sand layer was produced.

B. A quantity of Cabimas crude oil (viscosity 280 cs. at 25 C.) was mixed with silversand (permeability approximately 30 darcys), an oil sand being obtained the degree of saturation of which was 40%.

After the oil sand in the tube had been brought to the ignition temperature (300 C.) by electrical heating, air was introduced at a pressure of 10 atm. gauge at a rate of 650 litres per hour (measured under standard conditions). Owing to the combustion of oil the temperature rose to 550 C.; in the beginning the combustion was observed to proceed regularly.

After some time, however, the CO content of the gases produced, which was originally 8%, .slowly began to drop to approximately 1%; at the same time the temperature in the combustion zone was found to recede to below 400 C., which showed that there was no longer suflicient oil presentiin situ for maintaining the combustion.

Then, according to the invention, the upper part of the tube was closed so that the pressure increased at this place to 6.5 atm. gauge, while the pressure in the lower part was reduced to atmospheric pressure. This state was maintained for 10 minutes.

The pressure in the lower part was then again restored to 10 atm. gauge and the temperature rose. by approximately degrees Celsius, the CO content rising to 14%. This shows that oil was returned to the combustion zone and that the combustion was restarted.

After 1 /2 hours, during which time the combustion had progressed 35 cm., the temperature of the combustion front was again found to have dropped to below 400 C.; also the CO content was reduced. The above procedure was then repeated. Thereafter the combustion continued to progress regularly; the total yield of oil was 73%.

During the whole test the pressure drop over the tube was not more than a few atmospheres, so that there was no question of a stoppage through congestion of oil.

For the purpose of a broad comparison with conditions in practice, it may be added that a period of one minute in the model test approximately corresponds to a period of from some weeks to some months in carrying out the process in practice.

We claim as our invention:

1. A method of carrying out underground combustion in an oil-bearing formation for producing petroleum from said formation, said method comprising introducing into said formation through an input well a combustible gaseous mixture at a pressure greater than that existing in the formation, effecting combustion of said mixture with a portion of the oil in said formation adjacent said input well to produce combustion products which drive the toil-in said formation'toward an-output-well which isat a lower pressureto produce-a fluid flow=between the 'wells, -at-least once reversing; the fluid flow in said oilbearing formation prior to the-flame front reaching the output well by injecting a gas under pressure into said --output v/ellso-"that at least. a portion of the oil in the combustion zone of said formation moves -temporarily in .-a direction substantially opposite tothat prevailing during said first injection for a period suflicient to force combustible material back to theflarne front, and subsequentlyxagain reversing the fluid 'flowin said formation to its original condition,rwhereby all of the production products of said combustion are produced from said output well while the overall movement of said-combustion zone is from theinput well to the output'well.

.2. -A method of carrying out underground combustion ;in an oil-bearing formation for'producing petroleum from said formation, said method comprising introducing into .said formation through an input well a combustible gaseous mixturetat a pressure greater than that existing in the formation, efiecting combustion of I said mixture ;with

.a portion of the oil in said formation-adjacent said input 1 -well'to produce combustion products-which drive the oil insaid formationtoward an output well which is'at a "lower-pressure-to produce a fluid flow between the wells, measuring the pressure drop over theformation between the input and output Wells, reversing thefluid flow in said oil-bearing formation prior to the flame front reaching -the output-well after a continuing increase in pressure -drop'between the input and output wells is measured by injectinga; gas under pressure into ,saidoutput well so that at least a portion of the oil' in the combustion-zone of saidformation moves temporarily in a direction substantially opposite to that prevailing during said first injection for aperiod sufiicientt to force combustiblegmaterialback to the flame front, and subsequently again reversing the fluid flow in :said formation to its original condition, whereby all of the production products of said combustion are produced fromsaid output -well .-while the overall movement of said combustion zone is from the input well-to the outputwell.

-3. -A methodof carrying out undergroundcombustion in an oil-bearing formation for producing petroleum from said formation, said method comprisingrintroducing into said formation through an input well a combustible gasveous mixture at a pressure greater than that existing in the formation, effecting combustionof said mixture with a portion of the oil in said formation adjacent said input well to produce combustion products which drive the oil in said'formation toward'anoutput well which is at a lower pressure to produce a fluid flow between the Wells, measuring the combustion in intensity in said formation by analyzing the combustion gases produced from said output well, reversing the fluid flow in said oil-bearing formation prior to the flame front reaching the output well after a substantial decrease in the combustion intensity of said formation is measured by injecting a gas under pressure into said output well so that at least a portion of the oil in the combustion zone of said formation moves temporarily in a direction substantially opposite to that prevailing during said first injection for a period suflicient to force combustible material back to the flame front, and subsequently again reversing the fluid flow in said formation to its original condition, whereby all of the production products of said combustion are produced from said output well while the overall movement of said combustion zone is from the input well to the output well.

4. A method of carrying out underground combustion in an oil-bearing formation for producing petroleum from said formation, said method comprising introducing into said formation through an input well a combustible gaseous mixture at a pressure greater than that existing in the formation, effecting combustion of said mixture with a portion of the oil in said formation adjacent said input well to produce combustion products which drive -istatga lower pressure to produce said output 7 well so '6 the oil-in said formationtoward an outputwellwhieh a ,fluid .flow -between thewells, reversing the fluid flow in said oil-bearin iormation .prior to theflame from reaching the output well by discontinuing the injection of combustion gases into said input well and injecting a gas under pressure into that at; leasta portion of the oil in the combustion zone of said. formation moves temporarily in --a direction substantially opposite to thatprevailing during saidfirst injection for a period sufl'icient ,to force combustible material back: to the flame front, subsequently again reversing thefluid-sflow in said formation to its original condition by discontinuingtheinjectionof gas into said output-well and again injecting combustible gases intosaid input well, whereby all ,ofthe production products of said combustionare produced from said-output well while the overall movement of said combustion zone is from the input well to the output well.

5. A methodofcarrying-out'underground combustion in, an oil-bearingformation forprod-ucing petroleum from said formation, said method comprising introducing into said formation through .an input well a combustible gaseous mixture at a pressure greater than ,thatexisting -in the formation, effecting combustion of said mixture with a'portion of the oilinsaidtformation adjacent said input well to produce combustion products which drive the oil in said formation toward an outputwell which is at a lower pressure to produce a. fluid flow between the Wells, reversing the fluidflow in said oil-bearing formation prior to theflame front reaching the output well :by injecting-an-inert gas into vsaid ,output well-at -.a pressure; greater ,than the pressure of said input;.well so that at least aportion of the oil :in the combustion zone of saidformationmovestemporarilyin a direction sub- -stantiallyopposite,to that-prevailing during said first injection-for a period sufiicient to force combustible materialuback to the-flame front and subsequently again reversing thefluid flow in said formation to itsoriginal condition :by discontinuing injection ofmgas into saidoutput well, whereby all of the production products .of said combustion are produced from said output well while theoverall movement of saidcombuSt-ionzone is from the input wellto the output well.

6. A method of carrying out. undergroundfcombustion in an oil-bearing formation for producing petroleum from said formation, said method comprising introducing into said formation through an input well an oxygencontaininggas ata pressure greater than that existing -in-the formation, effecting combustion ofsaid .gas with a portion of the oil'in said formation adjacentsaid input ;,well;;to;produce combustion products which drive the oil in said formation toward an output well which is at a lower pressure to produce a fluid flow between the wells, at least once reversing the fluid flow in said oilbearing formation prior to the flame front reaching the output well by injecting a gas under pressure into said output well so that at least a portion of the oil in the combustion zone of said formation moves temporarily in a direction substantially opposite to that prevailing during said first injection for a period suflicient to force combustible material back to the flame front, and subsequently again reversing the fluid flow in said formation to its original condition, whereby all of the production products of said combustion are produced from said output well while the overall movement of said combustion zone is from the input well to the output well.

7. A method of carrying out underground combustion in an oil-bearing formation for producing petroleum from said formation, said method comprising introducing into said formation through an input well an oxygencontaining gas at a pressure greater than that existing in the formation, effecting combustion of said gas with a portion of the oil in said formation adjacent said input well to produce combustion products which drive the oil in said formation toward an output well which is at a the'inputand output wells,'reversing the fluid flow in said oil-bearing formation prior to the flame front reaching the output well after a continuing increase in pressure drop between the input and output wells is measured by injecting a gas under pressure into said output well so that at least a portion of the oil in the combustion zone of said formation moves temporarily in a direction substantially opposite to that prevailing during said first injection for a period suflicient to force combustible material back to theflame front, and subsequently again reversing the fluid flow in said formation to its original condition, whereby all of the production products of said combustion are produced from said output well while the overall movement of said combustion zone is from the input well to the output well.

8. A method .of carrying out underground combustion in an oil-bearing formation for producing petroleum from said formation, said method comprising introducing into said formation through an input well an oxygen-containing gas at a pressure greater than that existing in the formation, effecting combustion of said gas with a portion of the oil in said formation adjacent said input well to produce combustion products which drive the oil in said formation toward an output well which is at a lower pressure to produce a fluid flow between the wells, measuring the combustion in intensity in said formation by analyzing the combustion gases produced from said output well, reversing the fluid flow in said oil-bearing formation prior to the flame front reaching the output well after a substantial decrease in the combustion intensity of said formation is measured by injecting a gas under pressure into said output well so that at least a portion of the oil in the combustion zone of said formation moves temporarily in a direction substantially opposite to that prevailing during said first injection for a period sufficient to force combustible material back to the flame front, and subsequently again reversing the fluid flow in said formation to its original condition, whereby all of the production products of said combustion are produced from said output well while the overall movement of said combustion zone is from the input well to the output well.

9. A method of carrying out underground combustion in an oil-bearing formation for producing petroleum from said formation, said method comprising introducing into said formation through an input well an oxygen-containing gas at a pressure greater than that existing in the formation, effecting combustion of said gas with a portion of the oil in said formation adjacent said input well to produce combustion products which drive the oil in said formation toward an output well which is at a lower pr essure to produce afluid flow between the wells, reversing the fluid flow in said oil-bearing formation prior to the flame front reaching the output well by discontinuing the injection of combustion gases into said input well and injecting a gas under pressure into said output well so that at least a portion of the oil' in the combustion zone of said formation moves temporarily in a direction substantially opposite to that prevailing during said first injection for a period sufiicient to force combustible material back to the flame front, subsequently again reversing the fluid flow in said formation to its original condition by discontinuing the injection of gas into said output well and again injecting combustible gases into said input well, whereby all of the production products of said combustion are produced from said output well while the overall movement of said combustion zone is from the input well to the output well.

10. A method of carrying out underground combustion in an oil-bearing formation for producing petroleum from 'said formation, said method comprising introducing into said formation through an input well an oxygen-containing gas at a pressure greater than that existing in the formation, effecting combustion of said gas with a portion of the oil in said formation adjacent said input well to produce combustion products which drive the oil in said formation toward an output well which is at a lower pressure to produce a fluid flow between the wells, re versing the fluid flow in said oil-bearing formation prior to the flame front reaching the output well by injecting an inert gas into said output well at a pressure greater than the pressure of said input well so that at least a portion of the oil in the combustion zone of said formation moves temporarily in a direction substantially opposite to that prevailing during said first injection for a period suflicient to force combustible material back to the flame front, and subsequently again reversing the fluid flow in said formation to its original condition by discontinuing injection of gas into said output well, whereby all of the production products of said combustion are produced from said output well while the overall movement of said combustion zone is from the input well to the output Well.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD OF CARRYING OUT UNDERGROUND COMBUSTION IN AN OIL-BEASRING FORMATION FOR PRODUCING PETROLEUM FROM SAID FORMATION, SAID METHOD COMPRISING INTRODUCING INTO SAID FORMATION THROUGH AN INPUT WELL A COMBUSTIBLE GASEOUS MIXTURE AT A PRESSURE GREATER THAN THAT EXISTING IN THE FORMATION, EFFECTING COMBUSTION OF SAID MIXTURE WITH A PORTION OF THE OIL IN SAID FORMATION ADJACENT SAID INPUT WELL TO PRODUCE COMBUSTION PRODUCTS WHICH DRIVE THE OIL IN SAID FORMATION TOWARD AN OUTPUT WELL WHICH IS AT A LOWER PRESSURE TO PRODUCE A FLUID FLOW BETWEEN THE WELLS, AT LEAST ONCE REVERSING THE FLUID FLOW IN SAID OILBEARING FORMATION PRIOR TO THE FLAME FRONT REACHING THE OUTPUT WELL BY INJECTING A GAS UNDER PRESSURE INTO SAID OUTPUT WELL SO THAT AT LEAST A PORTION OF THE OIL IN THE COMBUSTION ZONE OF SAID FORMATION MOVES TEMPORARILY IN A DIRECTION SUBSTANTIALLY OPPOSITE TO THAT PREVAILING DURING SAID FIRST INJECTION FOR A PERIOD SUFFICIENT TO FORCE COMBUSTIBLE MATERIAL BACK TO THE FLAME FRONT, AND SUBSEQUENTLY AGAIN REVERSING THE FLUID FLOW IN SAID FORMATION TO ITS ORIGINAL CONDITION, WHEREBY ALL OF THE PRODUCTION PRODUCTS OF SAID COMBUSTION ARE PRODUCED FROM SAID OUTPUT WELL WHILE THE OVERALL MOVEMENT OF SAID COMBUSTION ZONE IS FROM THE INPUT WELL TO OUT WELL. 